The present invention relates to a fuser member, a method of fusing toner images in electrostatographic reproducing apparatus and a method for fabricating fuser members. More specifically, the present invention relates to a fuser member, preferably in the form of a fuser roll, pressure roll or release agent donor roll with a coating thereover, and which coating contains metal oxides like a copper oxide.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known. To fuse electroscopic toner material onto a support surface permanently by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, the thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90.degree. C. to about 200.degree. C. or higher depending upon the softening range of the particular resin used in the toner. It is undesirable, however, to increase the temperature of the substrate substantially higher than about 250.degree. C. because of the tendency of the substrate to discolor or convert into a fire, at such elevated temperatures, particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means, a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip affects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The referred to "hot offset" occurs when the temperature of the toner is increased to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
Particularly preferred fusing systems are comprised of a heated cylindrical fuser roll having a fusing surface which is backed by a cylindrical pressure roll forming a fusing nip therebetween. A release agent donor roll is also provided to deliver release agent to the fuser roll. While the physical and performance characteristics of each of these rolls, and particularly of their functional surfaces are not precisely the same depending on the various characteristics of the fusing system desired, the same classes of materials are typically used for one or more of the rolls in a fusing system in an electrostatographic printing system.
One fusing system involved the use of silicone elastomer fusing surfaces, such as a roll with a silicone oil release agent which could be delivered to the fuser roll by a silicone elastomer donor roll. The silicone elastomers and silicone oil release agents used in such systems are described in numerous patents and fairly collectively illustrated in U.S. Pat. No. 4,777,087 to Heeks et al. While highly successful in providing a fusing surface with a very low surface energy to provide excellent release properties to ensure that the toner is completely released from the fuser roll during the fusing operation, these systems suffer from a significant deterioration in physical properties over time in a fusing environment. In particular, the silicone oil release agent tends to penetrate the surface of the silicone elastomer fuser members resulting in swelling of the body of the elastomer causing major mechanical failure including debonding of the elastomer from the substrate, softening and reduced toughness of the elastomer causing it to chunk out and crumble, contaminating the machine and providing nonuniform delivery of release agent. Furthermore, as described in U.S. Pat. No. 4,777,087, additional deterioration of physical properties of silicone elastomers results from the oxidative crosslinking, particularly of a fuser roll at elevated temperatures.
A more recent development in fusing systems involves the use of fluoroelastomers as fuser members which have a surface with a metal containing filler, which interact with polymeric release agents having functional groups, which interact with the metal containing filler in the fluoroelastomer surface. Examples of these fusing systems, fusing members and release agents, are described in U.S. Pat. No. 4,264,181 to Lentz et al, U.S. Pat. No. 4,257,699 to Lentz and U.S. Pat. No. 4,272,179 to Seanor. Typically, the fluoroelastomers are (1) copolymers of vinylidenefluoride and hexafluoropropylene, and (2) terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene. Commercially available materials include VITON E430.TM., VITON GF.TM. and other VITON.TM. designations as Trademarks of E.I. DuPont de Nemours, Inc. as well as the FLUOREL.TM. materials of 3M Company. The preferred curing system for these materials is a nucleophilic system with a bisphenol crosslinking agent to generate a covalently crosslinked network polymer formed by the application of heat following basic dehydrofluorination of the copolymer. Exemplary of such a fuser member is an aluminum base member with a poly(vinylidenefluoride-hexafluoropropylene) copolymer cured with a bisphenol curing agent having lead oxide filler dispersed therein and utilizing a mercapto functional polyorganosiloxane oil as a release agent. In those fusing processes, the polymeric release agents have functional groups (also designated as chemically reactive functional groups) which interact with the metal containing filler dispersed in the elastomer or resinous material of the fuser member surface to form a thermally stable film which releases thermoplastic resin toner and which prevents the thermoplastic resin toner from contacting the elastomer material itself. The metal oxide, metal salt, metal alloy or other suitable metal compound filler dispersed in the elastomer or resin upon the fuser member surface interacts with the functional groups of the polymeric release agent. Preferably, the metal containing filler materials do not cause degradation of or have any adverse effect upon the polymeric release agent having functional groups. Because of this reaction between the elastomer having a metal containing filler and the polymeric release agent having functional groups, excellent release and the production of high quality copies are obtained even at high rates of speed of electrostatographic reproducing machines.
While the mechanism involved is not completely understood, it has been observed that when certain polymeric fluids having functional groups are applied to the surface of a fusing member having an elastomer surface with a metal oxide, metal salt, metal, metal alloy or other suitable metal compounds dispersed therein there is an interaction (a chemical reaction, coordination complex, hydrogen bonding or other mechanism) between the metal of the filler in the elastomer and the polymeric fluid having functional groups so that the polymeric release agent having functional groups in the form of a liquid or fluid provides an excellent surface for release which has an excellent propensity to remain upon the surface of the fuser member. Regardless of the mechanism, there appears to be the formation of a film upon the elastomer surface which differs from the composition of the elastomer and the composition of the polymeric release agent having functional groups. This film, however, has a greater affinity for the elastomer containing a metal compound than the toner and thereby provides an excellent release coating upon the elastomer surface. The release coating has a cohesive force which is less than the adhesive forces between heated toner and the substrate to which it is applied and the cohesive forces of the toner. The interaction between the functional group of the polymeric release agent and the metal of the elastomer containing metal leads to an overall diminution of the critical or high surface energy of the metal in the metal containing filler.
Preferred elastomers include the fluoroelastomers, such as fluoroelastomers of vinylidenefluoride based fluoroelastomers, which contain hexafluoropropylene and tetrafluoroethylene as comonomers. Two of the most preferred fluoroelastomers are (1) a class of copolymers of vinylidenefluoride and hexafluoropropylene known commercially as VITON A.TM. and (2) a class of terpolymers of vinylidenefluoride, hexafluoropropylene and tetrafluoroethylene known commercially as VITON B.TM.. VITON A.TM. and VITON B.TM. and other VITON.TM. designations are trademarks of E.I. DuPont de Nemours and Company. Other commercially available materials include FLUOREL.TM. of 3M Company, VITON GH.TM., VITON E60C.TM., VITON B 910.TM., and VITON E 430.TM.. The preferred curing system is a nucleophilic system with a bisphenol crosslinking agent to generate a covalently crosslinked network polymer formed by the application of heat following basic dehydrofluorination of the copolymer. The nucleophilic curing system also includes an organophosphonium salt accelerator. Some of the commercially available fluoroelastomer polymers which can be cured with the nucleophilic system are VITON E 60C.TM., VITON B 910.TM., VITON E 430.TM., VITON A.TM., VITON B.TM., and VITON GF.TM..
The use of polymeric release agents having functional groups which interact with a fuser member to form a thermally stable, renewable self-cleaning layer having superior release properties for electroscopic thermoplastic resin toners is described in U.S. Pat. Nos. 4,029,827 to Imperial et al., 4,101,686 to Strella et al. and 4,185,140 also to Strella et al. all commonly assigned to the assignee of the present invention. In particular, U.S. Pat. No. 4,029,827 is directed to the use of polyorganosiloxanes having mercapto functionality as release agents. U.S. Pat. Nos. 4,101,686 and 4,185,140 are directed to polymeric release agents having functional groups such as carboxy, hydroxy, epoxy, amino, isocyanate, thioether, and mercapto groups as release fluids.
While these fluoroelastomers have excellent mechanical and physical properties in that they typically have a long wearing life maintaining toughness and strength over time in a fusing environment, they can only be used with very expensive functional release agents and have to contain expensive interactive metal containing fillers. Typically, for example, the functional release agents are of the order of four times expensive as their nonfunctional conventional silicone oil release agents.
"Improving Release Performance of Viton Fuser Rolls", by Henry et al., Xerox Disclosure Journal, Volume 9, No.1, January/February 1984, discloses a fuser member made of a copolymer of vinylidenefluoride and hexafluoropropylene which has a tendency to react with the toner charge control agent producing increased crosslinking and thereby hardening as the double bonds of the fluoroelastomer become saturated to prevent further crosslinking by the addition of a silanic hydrogen compound, such as polymethylhydrosiloxane, to covalently bond the siloxane to the surface of this fluoroelastomer and thereby prevent further hardening, and in addition provide good release characteristics.
"Viton/RTV Silicone Fuser Release Overcoating", Ferguson et al., Xerox Disclosure Journal, Volume 11, No.5, September/October 1986, describes a fusing member wherein a fluoroelastomer such as a copolymer of vinylidenefluoride and hexafluoropropylene and an RTV Silicone Rubber are co-dissolved, co-sprayed and co-cured on an aluminum substrate to provide a uniform dispersion of silicone within the fluoroelastomer matrix. Such a fuser surface is described as having the mechanical strength of the fluoroelastomer and the release properties of the silicone, and may be used with traditional dimethyl silicone release fluids.
U.S. Pat. No. 4,853,737 to Heartly et al. describes a fuser roll comprising a cured fluoroelastomer containing pendant diorganosiloxane segments that are covalently bonded to the backbone of the fluoroelastomer. The siloxane is appended to the fluoroelastomer by adding to the composition to be cured a polydiorganosiloxane oligomer having functional groups such as phenoxy or amino groups to form the covalent bond. The fuser member preferably has a metal oxide containing filler to react with functional release agent.
In U.S. Pat. No. 5,166,031, the disclosure of which is totally incorporated herein by reference, there is illustrated a fuser member comprising a supporting substrate having an outer layer of a volume grafted elastomer which is a substantially uniform integral interpenetrating network of a hybrid composition of a fluoroelastomer and a polyorganosiloxane, said volume graft having been formed by dehydrofluorination of said fluoroelastomer by a nucleophilic dehydrofluorinating agent, followed by addition polymerization by the addition of an alkene or alkyne functionally terminated polyorganosiloxane and a polymerization initiator, and wherein said fluoroelastomer can be selected from a group consisting of poly(vinylidene fluoride-hexafluoro-propylene) and poly(vinylidene fluoridehexafluoropropylene-tetrafluoroethylene), and wherein said polyorganosiloxane is of the formula ##STR1## where R is an alkyl, alkenyl or aryl having less than 19 carbon atoms or an aryl group substituted with an amino, hydroxy, mercapto or alkyl or alkenyl group having less than 19 carbon atoms, the functional group A is an alkene or alkyne, preferably with 2 to 8 carbon atoms. Disadvantages that may be associated with the aforementioned fusing member include its lack of substantial toughness, that is for example this member can be relatively less wear resistant, and thus reduced fuser life; the amount of polyorganosiloxane in the graft coating may not, in certain situations, be sufficient that it can release effectively from paper without the use of a release fluid; inadequate release characteristics with mercapto functional oils, and thus adding small amounts of copper oxide, from about 2 to about 7 percent by weight provide sufficient anchoring sites for the functional oil to enable adequate release to paper; amounts higher than 7 percent and lower than 2 percent cause, for example, release problems. There is thus a need to overcome or minimize the aforementioned disadvantages and more specifically a need to reduce the amount of copper oxide that is added to fuser coatings to levels where the fuser surface does not degrade, and desirable adequate release characteristics are obtained for extended time periods during use in, for example, a xerographic imaging or printing apparatus. These and other needs can be achieved with the fuser members of the present invention.